1. Field of the Invention
The invention relates to the field of metallurgy and particularly to the field of titanium base alloys.
2. Description of the Prior Art
In the development of titanium alloys, the main emphasis has been placed upon obtaining alloys which have good mechanical and physical properties (such as strength, toughness, ductility, density, corrosion resistance, etc.) for specific applications. In general the fabricators of finished parts have had to adapt their processing (machining, welding, forging, forming, etc.) to meet the requirements of the alloy.
One relatively new process which fabricators have used to form parts from titanium alloys is superplastic forming. As described in U.S. Pat. No. 4,181,000, the alloy is stressed at a strain rate and at a temperature which causes it to flow large amounts without necking down and rupturing. The ability of some alloys to flow under these conditions is a property called superplasticity. This property is measured using stress strain tests to determine the alloy's strain rate sensitivity, according to the classical equation: ##EQU1## where: m=strain rate sensitivity,
.sigma.=stress, PA1 .epsilon.=strain rate, and PA1 K=constant,
The higher the value of m, the more superplastic the alloy being measured.
Fortunately, most titanium alloys exhibit superplastic properties under the proper conditions of stress and temperature. This fact is a fortunate happenstance because the alloys were formulated without any concern for, or even awareness of, the superplastic formability. As a result, prior art titanium alloys do not have optimum superplastic properties.
An example of such a prior art titanium alloy is an alloy designated as Ti-6Al-4V which is described in U.S. Pat. No. 2,906,654. This alloy is widely used because of its good properties and good fabricability. It is superplastic, having a maximum strain rate sensitivity (.sup.m max) at 1600.degree. F. in the range of 0.62 to 0.68.